The invention relates to an optical fiber cable of the kind in which the optical fibers are located in grooves which extend helically either in the outer surface of a cylindrical body which extends along the length of the cable or in the inner surface of a jacket directly surrounding a cylindrical body. The grooves may wind in one direction or they may periodically change direction. The optical fibers are disposed in the grooves with such an excess length that elongation of the cable, which may occur, for example, during laying of the cable or by thermal expansion, can be compensated for.
Cables in which the grooves are arranged in the outer surface of a central cylindrical body are known, generally. A further element which is, for example, provided in the center of the cylindrical body may be present for the purpose of absorbing tensile loads. One or more metal wires or synthetic resin material wires (for example Kevlar) or glass fibers may, for example, be used for this purpose. The cylindrical body may also be a synthetic resin material.
The grooves in the cylindrical body may extend helically along the full length of the cable into one direction only, or they may repeatedly change their direction after one or more turns or part thereof.
The grooves may have any cross-sectional shape, for example a rectangular shape. However, there must be sufficient room in the grooves that each fiber can take a position in a groove without being mechanically loaded in a manner which detrimentally affects the optical quality of the fiber. This means that there must be sufficient room to offset positive or negative changes in the ratio between the length of the cable and the length of the fibers. One or more layers of a synthetic resin material may be applied around the cylindrical elongate body, for example in the form of wound tapes and one or more jackets and reinforcing elements, wire armourings etc., as is customary in the cable industry to protect the cables from moisture and mechanical stress. The grooves may be filled along their full length with a material which guarantees that they will be water tight along the entire length of the cable. For example, the grooves may be filled with a paraffin jelly, silicone oil or silicone grease.
With a cable of this nature there is the risk that due to vibration of a portion of the cable, for example due to traffic, or due to the orientation of a portion of the cable, for example due to a vertical or sloping orientation, the fibers may move along the length of the cable and the originally uniformly distributed excess length assembles in one or more places, for example the lowest place(s). This is disadvantageous, because changes in the length of the cable, for example as a result of an increase in temperature, may result in a mechanical loading of the optical fibers in those places where there is no longer any excess length. Mechanical loading may also occurs when the temperature falls in those places in the cable where there is no longer any excess room in the groove to take up the excess length.